Plasminogen activator inhibitor 1 (PAI-1) is the primary inhibitor of plasminogen activation and vascular fibrinolysis in vivo, and elevations in PAI-1 appear to compromise normal fibrin clearance mechanisms and promote pathological thrombosis. Although there is a strong correlation between the elevated PAI-1 in human obesity and the increased risk for atherothrombotic disease, little is known about the origin of this plasma inhibitor in obesity, or about the signals that control its biosynthesis. Preliminary Studies demonstrate that PAI-1 biosynthesis by cultured endothelial cells (ECs) is stimulated by leptin, a hormone known to regulate appetite. In Aim 1, competitive RT-PCR and in situ hybridization will be employed to test the hypothesis that leptin is a clinically important regulator of PAI-1 and of EC-mediated vascular fibrinolysis in vivo. The possibility that leptin regulates the expression of other hemostatic genes by ECs (e.g., tissue factor; TF) will be examined, and the mechanism and signaling pathways by which leptin regulates PAI-1 in ECs will be investigated. Preliminary Studies show that deletion of the PAI-1 gene from genetically obese (ob/ob) mice improves their hyperglycemia and hyperinsulinemia, and lowers tumor necrosis factor alpha (TNF- alpha) mRNA levels in their adipose tissue. In Aim 2, the phenotype of these mice will be further characterized. The effect of PAI-1 deletion on other genes known to be elevated in ob/ob mice (e.g., TF and transforming growth factor-beta), on the renal pathology and thrombotic tendency of ob/ob mice, and on the phenotype of other obese mice (e.g., db/db; diet-induced), will be investigated. The hypothesis that the decrease in glucose and insulin results from the decreased TNF-alpha will be tested. Preliminary Studies also show that PAI-1-deficient and vitronectin-deficient mice develop unstable thrombi in their carotid arteries following chemical (feCl3) injury. In Aim 3, the contribution of t-PA and u-PA to the formation of such thrombi will be investigated, and the vascular response to injury in lean and obese mice will be compared. The hypothesis that the elevated PAI-1 and TF levels in ob/ob mice promote a shortened time to arterial thrombosis after injury and an altered rate of neointima formation, will be examined. Finally, the significance of the binding of leptin to components of the injured vessel wall will be investigated. These studies will provide insights into the nature of factors that regulate vascular thrombosis, fibrinolysis, and neointima formation.